Gasoline direct injection (GDI) system are becoming a popular technology in spark ignition engines to harvest the advantage of both higher thermodynamic efficiency and less engine-out emissions. The mixture formation in a direct injection engines is an outcome of careful matching of the air-fuel mixture and accurate fuel injection process. The injection rate is a vital parameter in understanding the injector parameters, spray characteristics, and in-turn its effect on air-fuel mixture and combustion process. Therefore, this study is focused on developing an injection rate rig based on momentum flux method and then measure the injection rate and characterize the high-pressure GDI injector custom made for research purpose. The injection rate was measured over a wide range of injection pressure and total mass injected. The injection rate measurement was complemented with high-speed imaging to correct the hydraulic start and end of injection to the true value. Then the hydraulic delays and discharge coefficient were characterized for this GDI injector.